MRAM with sidewall protection and method of fabrication

ABSTRACT

BEOL memory cells are described that include one or more sidewall protection layers on the memory device (including, for example, an MTJ element) deposited prior to interconnect via etching to prevent the formation of electrical shorts between layers. One embodiment uses a single layer sidewall protection sleeve that is deposited after the memory device has been patterned. The layer material is vertically etched down to expose the upper surface of the top electrode while leaving a residual layer of protective material surrounding the rest of the memory device. The material for the protection layer is selected to resist the etchant used to remove the first dielectric material from the via in the subsequent interconnect process. A second embodiment uses dual-layer sidewall protection in which the first layer covers the memory element is preferably an oxygen-free dielectric and the second layer protects the first layer during via etching.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of co-pending U.S. patentapplication bearing Ser. No. 13/136,454 with Filing Date of Aug. 1,2011, and entitled “MRAM with Sidewall Protection and Method ofFabrication,” which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to semiconductorBack-End-Of-Line (BEOL) memories and particularly to Magnetic RandomAccess Memory (MRAM) and particularly to protecting the memory elementsfrom being shorted during the interconnect process.

DESCRIPTION OF THE PRIOR ART

The BEOL memories such as RRAM (Resistive Random Access Memory), PRAM(Phase Change Random Access Memory), MRAM have a resistive device as amemory element. Because high speed access and non-volatility at poweroff are promised by these devices, they may replace existing memoriesand create new markets.

The memory device consists of a top electrode, a bottom electrode andthe resistive memory element in between. The bottom electrode isconnected to a control device such as a transistor or a diode. After thelayers of the memory device have been patterned, the top electrode isconnected to a bit line in a series of process steps collectivelyreferred to as BEOL process which creates metal interconnect wires thatare insulated by dielectric material. Herein arises a problem with theprior art that can result in an electrical short being formed betweenthe top and bottom electrodes during fabrication. The design trend forthe resistive device is to scale down minimum feature size to minimizethe cell area. The BEOL feature size is generally larger than theminimum feature size defined in FEOL (Front End Of Line) process.Therefore, when using a via hole to interconnect the resistive device tothe bit line, there is no margin for the bit line to land on the topelectrode. The sidewall of the memory element would be exposed by thevia etch process. If the etching of the via reaches to the sidewall ofthe bottom electrode, then the subsequent metal deposition in the bitline interconnect process causes a short between the top electrode tothe bottom electrode, thereby destroying the functionality of thedevice. This failure condition that can arise during prior artprocessing is illustrated FIG. 1.

The resistive device of MRAM is a MTJ (Magnetic Tunnel Junction)including a free layer, a fixed layer and a barrier layer in between. Amagnetic moment of the free layer is manipulated to parallel orantiparallel to the fixed layer by applying an electric current. Whetherthe magnetic vector of the free layer is parallel or antiparallel to thefixed layer determines the low or high resistance state of the MTJ. Thetwo resistance states are defined as memory state “0” or “1”. Therefore,an electrical short that could happen when the via etch reaches down tothe level of the barrier layer is a severe issue in this device.

In published US patent application 20100181654 by Fujiwara, et al. (Jul.22, 2010) an insulating film, which will be called a borazinic filmherein, for a semiconductor device is described. The film is describedas having low permittivity, a low leak current, high mechanicalstrength, stability over time, and excellent water resistance. Theprocess for forming the film uses a carrier gas and a raw material gas,which has borazine skeletal molecules. The insulating film includescross-linked borazine skeletal molecules and is said to have bothinorganic or organic properties.

SUMMARY OF THE INVENTION

Embodiments of the invention include one or more protection layersdeposited on the sidewall of the memory device (including, for example,an MTJ element) prior to interconnect via etching to protect thesidewall during the via etching and prevent the formation of electricalshorts between the top and bottom electrodes. The invention isapplicable to MRAM and other BEOL memories. Embodiments of the inventiondisclose a MTJ MRAM memory cell having one or more sidewall protectionlayers on a memory device sidewall and the fabrication method thereof.Two embodiments are described.

The first embodiment uses a single layer sidewall protection sleeve thatis deposited after the memory device has been patterned. The bulkmaterial for the protection sleeve layer is deposited and thenvertically etched down to expose the upper surface of the top electrodewhile leaving a residual layer of protective material surrounding therest of the memory device. The material for the protection sleeve layeris selected to resist the etchant used to remove the dielectric materialfrom the via in the subsequent interconnect process. The interconnectprocess can be performed in the conventional manner.

The second embodiment uses dual-layer sidewall protection sleeve (orequivalently dual sleeves) in which the first layer covers the sidewallsof the memory element and the bottom electrode and the second layerprotects the first layer during the via etch process. The first layer ofthe sidewall protection sleeve is formed in the same manner as describedfor the single layer in the first embodiment. However, the material forthe first layer in this second embodiment is preferably an oxygen-freedielectric and does not need to be resistant to the etchants used duringthe interconnect process. After the first sidewall protection layer isvertically etched to expose the upper surface of the top electrode, thebulk material for the second layer of the sidewall protection sleeve isdeposited over the device(s) on the wafer, then it is vertically etchedto again expose the upper surface of the top electrode while leavingresidual material on the sidewall. The material for the secondprotection layer is selected to resist the etchant used to remove theetch-stop dielectric material from the via in the subsequentinterconnect process. The interconnect process can be performed in theconventional manner.

In either the first or second embodiments a single layer or a dual layeretch stop layer structure can be deposited over the wafer after thesidewall protection sleeve has been formed and before the inter-layerdielectric (ILD) is deposited. An advantage of the alternativeembodiment with the dual layer etch stop layer structure is that ithelps address the problem of thickness variations of ILD after CMPplanarization across a wafer and between wafers. The material for thetop etch stop layer is selected to have high selectivity during etchingof the ILD so that the etching depth reliably stops in the top etch stoplayer even when the ILD is thinner than average.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a failure condition that can arise during prior artprocessing of a memory device where the etching of the interconnect viahole has exposed the bottom electrode resulting in a short between thetop and bottom electrodes when the metal for the bit line interconnectis deposited.

FIG. 2 illustrates a cross sectional view, perpendicular to thesubstrate surface, of a completed memory cell according to a firstembodiment of the invention with a single layer sidewall protectionsleeve.

FIGS. 3, 4A, 4B, 5A and 5B illustrate cross sectional views of selectedstages of the fabrication process for a memory cell according to thefirst embodiment of the invention.

FIG. 6 illustrates a cross sectional view of a completed memory cellaccording to a second embodiment of the invention with a dual-layersidewall protection sleeve.

FIGS. 7 and 8 illustrate cross sectional views of selected stages of thefabrication process for a memory cell according to the second embodimentof the invention with a dual-layer sidewall protection sleeve.

FIG. 9 illustrates a cross sectional view of a selected stage of thefabrication process for a memory cell according to an alternativeembodiment of the second embodiment of the invention with a dual-layersidewall protection sleeve and dual layer etch stop layer structure.

FIG. 10 illustrates a cross sectional view of a selected stage of thefabrication process for a memory cell according to an alternativeembodiment of the first embodiment of the invention with a single layersidewall protection sleeve and dual layer etch stop layer structure.

FIG. 11 illustrates a cross sectional view of a completed memory cellaccording to the alternative embodiment of the first embodiment of theinvention with a single layer sidewall protection sleeve and a duallayer etch stop layer structure.

FIG. 12 illustrates a cross sectional view of a completed memory cellaccording to the alternative embodiment of the second embodiment of theinvention with a dual-layer sidewall protection sleeve and a dual layeretch stop layer structure.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the embodiments, reference is made tothe accompanying drawings that form a part hereof, and in which is shownby way of illustration of specific embodiments in which the inventionmay be practiced. It is to be understood that other embodiments may beutilized without departing from the scope of the present invention. Itshould be noted that the figures discussed herein are not drawn to scaleand thicknesses of lines are not indicative of actual sizes. The crosssection view in the figures is generally taken through the approximatecenter the memory cell in a plane perpendicular to the substrate exceptwhere otherwise noted. Although only one cell is shown in the figures,the method may used for the simultaneous fabrication of a many cells ona wafer according to standard techniques.

FIG. 2 illustrates a cross sectional view of a completed memory cellaccording to a first embodiment of the invention wherein the sidewallsof the memory element and bottom electrode are covered by sidewallprotection sleeve 200. The material for sidewall protection sleeve 200is selected to have a relative low etch rate in the etching gas ambientfor dielectric material of etch stop layer 210 and is selected to be,for example, aluminum oxide, a borazinic film, silicon nitride,nitrogen-doped silicon carbide. A low dielectric constant borazinic filmsuitable for general MRAM applications can be deposited by a CVD systemsuch as the MAPLE (Multi Application PLasma Equipment) CVD system ofMitsubishi Heavy Industries.

The sidewall protection layer 200 conforms to the shape of the patternedmemory device and in this embodiment is an open-topped elliptical coneshape. After describing the completed device, the process of fabricatingthe device will be described and illustrated.

The memory device includes a top electrode 120, a bottom electrode 100and an MTJ element formed by fixed ferromagnetic layer 112, freeferromagnetic layer 116 and barrier layer 114. Multi-layer structurescan be substituted for single free and fixed layers in the MTJ as isknown in the art. The bottom electrode can be connected to a controldevice like a transistor or diode (not shown) in the standard manner.The top electrode is electrically connected by metal bit lineinterconnect 300, which is typically copper. The via area of the bitline interconnect 300 around the centrally located memory device isformed by etching away the dielectric etch stop layer 210 down below theplane of the upper surface of the top electrode, then refilling withmetal as part of the interconnect process. Because the sidewallprotection sleeve 200 extends the entire distance from the lower edge ofthe bottom electrode up onto the sides of the top electrode, the etchingdepth for the interconnect via in the dielectric etch stop layer 210 isless critical than in prior art designs. As long as the via exposes theupper surface of the top electrode, the depth of the via below the planeof the upper surface is not critical. The via could be etched all theway down the bottom electrode without causing a short failure. Theinvention, therefore, allows for higher yields even with inevitablevariations in the etching depth during fabrication. The etch depth forthe via in FIG. 2 is shown at approximately the midpoint of the verticalheight of the layer stack for the memory device as an example andvariations in the depth are to be expected.

The dielectric layer 220 is an inter-layer dielectric (ILD). ILD layer220 is preferably silicon oxide, which works well for CMP planarization.

An etch-stop layer 210 under ILD 220 works as an etch-stop layer duringetching of the ILD 220. The dielectric material for etch stop layer 210is selected to have a relatively slow etching rate in the etchingambient gas used for layer 220 etching. A dielectric material isselected for the protection sleeve 200 to be resistant during etching oflayer 210. Therefore, the protection sleeve 200 should be a differentmaterial than layer 210. In embodiments a material such as aluminumoxide or a borazinic film is selected for the sidewall protection sleeve200, because it has slow etching rate in the typical ambient gas forlayer 210 etching. The sidewalls of the memory element and the bottomelectrode are covered by sidewall protection sleeve 200, but the uppersurface of the top electrode is left exposed to the bit lineinterconnect 300. The sidewall protection sleeve 200 as shown extendsalmost to the plane of the upper surface of the top electrode and,therefore, covers most of the sidewall of the top electrode.

The cross section view in FIG. 2 and the other figures is taken throughthe approximate center the memory cell in a plane perpendicular to thesubstrate. In a plan view (not shown) parallel to the substrate surfacethe top electrode 120 and the other layers in the MTJ are generallyelliptically shaped and, therefore, the sidewall protection sleeve 200,which conforms to the shape of the MTJ will typically be an ellipticallyshaped concentric band. However, the shape of the MTJ is not criticalfor any of the embodiments of the invention, because the sidewallprotection sleeve will conform to whatever shape the MTJ has. Thus, thesidewall protection sleeve 200 is a generally a sleeve or ring ofmaterial that encircles the sidewalls of the layers of the memoryelement and the bottom electrode, separates these layers from contactwith the surrounding metal material of bit line interconnect 300, andthereby electrically insulates the sidewalls of these layers.

The process of fabricating a memory cell according to the firstembodiment invention will be described starting with FIG. 3. A verticalcross sectional illustration after patterning the memory element stackis shown in FIG. 3. The bottom electrode layer 100, MTJ layer stack 112,114, 116 and the top electrode layer 120 are deposited in sequence. Thefilm stack is then patterned using conventional photolithography andvertical etching.

Next a layer 200′ is deposited over the film stack as shown in FIG. 4A.This layer provides the bulk material for the sidewall protectionstructures 200 after controlled etching. The layer 200′ is verticallyetched until the top electrode 120 is exposed, which leaves residualmaterial on the sidewalls as shown in FIG. 4B forming the sidewallprotection sleeve 200. The exposed upper surface of top electrode 120will be connected to the bit line in later process steps.

As shown in FIG. 5A, etch stop layer 210 is then deposited over thepatterned film stack. Next the ILD oxide layer 220 is deposited. The ILDlayer 220 is then planarized by chemical-mechanical polishing (CMP) asshown in FIG. 5A. From the state shown in FIG. 5A, conventionalinterconnect processing such as a Dual Damascene process is used. Duringthe interconnect process, the sidewalls of the memory element and thebottom electrode are protected with the sidewall protection sleeve 200.An exemplary Dual Damascene process can include multiple etch steps. Forexample, in one etch, using a photoresist mask defining the via, theexposed portion of the ILD layer 220 overlying the etch stop layer 210is removed. A subsequent etch removes the etch stop layer 210 from thevia area around the memory element stack. During the etching, the memoryelement stack is protected by sidewall protection sleeve 200 which isresistant to etching ambient used to remove layer 210. The result of thesequence of etching steps is shown in FIG. 5B. As noted above the viaetching depth can be shallower or deeper than shown, with therequirement being that the upper surface of the top electrode isexposed. The remainder of the interconnect process is performedincluding depositing a metal, e.g., copper, over the wafer to form thebit line connection to the top electrode. The result is the memory cellhaving a sidewall protection sleeve embodiment of the invention as shownin FIG. 2.

In an alternative embodiment illustrated in FIG. 10, function of etchstop layer 210 is performed by sequentially deposited bottom and topetch stop layers 211, 212. The stage of the process illustrated in FIG.10 is comparable to the one illustrated in FIG. 5A but with two layers212, 211 replacing single layer 210. FIG. 11 illustrates a crosssectional view of a completed memory cell according to the alternativeembodiment of the first embodiment of the invention with a single layersidewall protection sleeve and a dual layer etch stop layer structure.The combined thickness of the two etch stop layers 212, 211 iscomparable to that of the single etch stop layer 210. An advantage ofthis alternative embodiment is that it helps address the problem ofthickness variations of ILD 220 after CMP planarization across a waferand between wafers. The material for the top etch stop layer 212 isselected to have high selectivity during etching of ILD 220 so that theetching depth reliably stops in the top etch stop layer 212 even whenthe ILD 220 is thinner than average. Thus the added etch resistanceprovided by top etch stop layer 212 reduces variation resulting from theILD etching step caused by the thickness variation of ILD 220. Theresult is that variation of residual thickness of the layers over thememory element is considerably smaller compared that of ILD 220.

The dielectric material for top etch stop layer 212 is selected to havea relatively slow etching rate in the etching ambient gas used for layer220 etching so that etching stops in the layer 212 controllably. Forexample, the material for top etch stop layer 212 can be siliconnitride, nitrogen-doped silicon carbide, aluminum oxide or a borazinicfilm. The material for bottom etch stop layer 211, for example, can besilicon nitride, silicon oxide, Silicon carbide and so on is combinedwith a material of bottom top etch stop layer 212. Subsequent etching ofthe dual etch stop layer embodiment is well controlled by selecting anetching ambient that etches the material in top etch stop layer 212 at arate that is equal to or faster than the material in bottom etch layer211.

Top etch stop layer 212 should not be the same material as ILD layer220. The 3 materials for layers 220, 212, 211 should be selected withregard to each other. Materials are selected using the followingcriteria. The etch rate of top etch stop layer 212 in ILD layer 220etching ambient is much smaller (slower) than ILD layer 220. Thiscriterion is the same as for the single etch stop layer 210 embodiments.The etch rate of top etch stop layer 212 in bottom etch stop layer 211etching ambient is comparable or faster than for top etch stop layer212. Therefore, bottom etch stop layer 211 can be a same material as topetch stop layer 212.

The following combinations of material are preferred:

-   -   A. Top etch stop layer 212=(nitrogen-doped silicon carbide,        aluminum oxide or a borazinic film) with bottom etch stop layer        211=(silicon nitride, or silicon oxide);    -   B. Top etch stop layer 212=(silicon nitride) with bottom etch        stop layer 211=(silicon nitride, or silicon oxide).

An exemplary embodiment can use a low k oxide such as silicon oxide forILD 220, nitrogen-doped silicon carbide for top etch stop layer 212, andsilicon nitride (low temperature) for bottom etch stop layer 211.

A cross sectional view of the second embodiment of the invention with adual-layer sidewall protection sleeve is illustrated in FIG. 6. Thememory element is the same as in the first embodiment. A first layer 230of sidewall protection is preferably an oxygen free dielectric to avoidoxidation of the MTJ stack and can be patterned as described above forthe first embodiment. The inner sidewall protection layer 230 is incontact with and covers the sidewall of the memory element. As in thefirst embodiment, the sidewall protection layer 230 conforms to theshape of the patterned memory element which in this embodiment is anopen-topped cone.

After the inner sidewall protection layer 230 has been etched down toexpose the upper surface of the top electrode, the second sidewallprotection layer 240 is deposited over and covers the inner sidewallprotection layer 230. For the outer sidewall protection layer 240 amaterial having a relatively slow etching rate in etching ambient gasused to etch layer 210. The material can be selected to be aluminumoxide or a comparable material. The upper surface of top electrode 120is not covered with either of the sidewall protection layers to allowconnection to the bit line interconnect 300 for standardinterconnection.

The initial steps in a process according to second embodiment of theinvention are generally the same as for the first embodiment, and thefirst sidewall protection layer 230 can be patterned as described abovefor layer 200 in the first embodiment. However, the material for theinner sidewall protection layer 230, is preferably an oxygen freedielectric such as silicon nitride or a silicon carbide. A bulkdielectric material 240′ is deposited over the wafer and innerprotection layer 230 as shown in FIG. 7 which will be patterned into theouter sidewall protection layer 240. The material for the outer sidewallprotection layer 240 is preferably aluminum oxide or borazinic film or acomparable material. The outer sidewall protection layer 240 protectsthe inner sidewall protection layer 230 during the interconnect process.The outer sidewall protection layer 240 is vertically etched until topof the memory element is exposed to allow interconnection with the bitline as shown in FIG. 8. The process continues in the conventionalmanner after the outer sidewall protection layer 240 is verticallyetched. Etch stop layer 210 and ILD oxide layer 220 are depositedsequentially. ILD 220 is planarized by CMP as shown in FIG. 8.

The etch stop layer 210 in the second embodiment as shown in FIG. 8 canbe replaced by a dual layer as described above for the first embodiment.A dual etch stop layer consisting of a top layer 212 and a bottom layer211 is illustrated in FIG. 9. The stage of the process illustrated inFIG. 9 is comparable to that shown in FIG. 8. FIG. 12 illustrates across sectional view of a completed memory cell according to thealternative embodiment of the second embodiment of the invention with adual-layer sidewall protection sleeve and a dual layer etch stop layerstructure.

It is followed by conventional interconnect processing such as a DualDamascene process. After this processing, a memory cell having dualsidewall protection layers is completed as shown in FIG. 6. During theinterconnect process, the outer sidewall protection layer 240 protectsthe inner sidewall protection layer 230 covering the sidewall of thememory element and thereby protects the memory element.

1. A memory cell comprising: a memory device disposed centrally in thememory cell, the memory device including a top electrode; a memoryelement having a plurality of layers; and a bottom electrode; a sidewallprotection sleeve of dielectric material disposed around sides of thememory element and the bottom electrode; a metal bit line interconnectin electrical contact with an upper surface of the top electrode, themetal bit line interconnect extending down below a plane of the uppersurface of the top electrode and making contact with the sidewallprotection sleeve; a bottom etch-stop layer surrounding a portion of themetal bit line interconnect making contact with the sidewall protectionsleeve, the etch-stop layer consisting of a dialectic material; and atop etch-stop layer consisting of a dialectic material deposited on thebottom etch-stop layer, the top etch-stop layer being separated fromcontact with the sidewall protection sleeve.
 2. The memory cell of claim1 wherein the bottom etch-stop layer is silicon nitride, silicon oxide,or silicon carbide.
 3. The memory cell of claim 1 wherein the topetch-stop layer is silicon nitride, nitrogen-doped silicon carbide,aluminum oxide or a borazinic film.
 4. The memory cell of claim 1wherein the sidewall protection sleeve includes a layer of aluminumoxide.
 5. The memory cell of claim 1 wherein the sidewall protectionsleeve includes a layer of a borazinic film.
 6. The memory cell of claim1 wherein the sidewall protection sleeve substantially covers a sidewallof the top electrode.
 7. The memory cell of claim 1 wherein the sidewallprotection sleeve includes an inner layer of a first dielectric materialand an outer layer of a second dielectric material.
 8. The memory cellof claim 7 wherein the inner layer is an oxygen-free dielectric.
 9. Thememory cell of claim 8 wherein the outer layer is aluminum oxide. 10.The memory cell of claim 8 wherein the outer layer is a borazinic film.11. A method for fabricating a memory cell comprising: depositing astack of layers for a memory device including a top electrode, a memoryelement and a bottom electrode; patterning the stack of layers for amemory device to expose sidewalls of the layers; depositing a layer ofdielectric material over the memory device and onto the sidewalls;vertically etching the layer of dielectric material to remove thedielectric material from an upper surface of the top electrode whileleaving a sleeve of dielectric material covering the sidewalls of thememory element and the bottom electrode; depositing a bottom etch-stoplayer of a dielectric material over the memory device and the sleeve ofdielectric material; depositing a top etch-stop layer of a dielectricmaterial over the bottom etch-stop layer; depositing an inter-layerdielectric layer over the top etch-stop layer; forming a via for a metalbit line interconnect by etching away a selected portion of theinter-layer dielectric layer, the top etch-stop layer and the bottometch-stop layer to expose an upper surface of the top electrode and atleast a portion of the sleeve of dielectric material; and forming ametal bit line interconnect in contact with the top electrode and thesleeve of dielectric material.
 12. The method of claim 11 wherein thebottom etch-stop layer is silicon nitride, silicon oxide, or siliconcarbide.
 13. The method of claim 11 wherein the top etch-stop layer issilicon nitride, nitrogen-doped silicon carbide, aluminum oxide or aborazinic
 14. The method of claim 11 wherein an etchant used to removethe selected portion of the bottom etch-stop layer removes the bottometch-stop layer at a faster rate than the sleeve of dielectric material.15. The method of claim 11 wherein the dielectric material in the sleeveis aluminum oxide.
 16. The method of claim 11 wherein the dielectricmaterial in the sleeve is aluminum oxide.
 17. The method of claim 11wherein the dielectric material in the sleeve is a borazinic film.
 18. Amethod for fabricating a memory cell comprising: depositing a stack oflayers for a memory device including a top electrode, a memory elementand a bottom electrode; patterning the stack of layers for a memorydevice to expose sidewalls of the layers; depositing a first layer of afirst dielectric material over the memory device and onto the sidewalls;vertically etching the first layer of a first dielectric material toremove the first dielectric material from an upper surface of the topelectrode while leaving a first sleeve of first dielectric materialcovering the sidewalls of the memory element and the bottom electrode;depositing a second layer of a second dielectric material over the topelectrode and the first sleeve covering the sidewalls; verticallyetching the second layer of the second dielectric material to remove thesecond dielectric material from the upper surface of the top electrodewhile leaving a second sleeve of the second dielectric material coveringthe first sleeve; depositing a bottom etch-stop layer of a dielectricmaterial over the memory device and the second sleeve of dielectricmaterial; depositing a top etch-stop layer of a dielectric material overthe bottom etch-stop layer; depositing an inter-layer dielectric layerover the top etch-stop layer; forming a via for a metal bit lineinterconnect by etching away a selected portion of the inter-layerdielectric layer, the top etch-stop layer and the bottom etch-stop layerto expose an upper surface of the top electrode and at least a portionof the sleeve of dielectric material; and forming a metal bit lineinterconnect in contact with the top electrode and the second sleeve ofdielectric material.
 19. The method of claim 18 wherein form an etchantused to remove the selected portion of the bottom etch-stop layerremoves the bottom etch-stop layer at a faster rate than the secondsleeve of dielectric material.
 20. The method of claim 18 wherein thesecond dielectric material in the second sleeve is aluminum oxide andthe top etch-stop layer is silicon nitride or nitrogen-doped siliconcarbide.
 21. The method of claim 18 wherein the first dielectricmaterial is an oxygen-free dielectric.
 22. The method of claim 18wherein the second dielectric material is aluminum oxide.
 23. The methodof claim 18 wherein the second dielectric material is a borazinic film.